The present invention relates to a frequency modulator.
A prior art frequency modulator has been constructed in a manner such as shown in FIG. 1. As shown, it comprises an oscillator circuit of a multi-vibrator type.
The circuit comprises a resistor R1 connected at one end to a supply voltage and at its other end to a first side of a tank circuit comprising an inductor L1, a capacitor C1, a resistor R2, a diode D1, and a diode D2, all connected in parallel with one another. The diodes D1 and D2 are connected in a mutually inverted fashion. A second side of the tank circuit is connected to the base of an NPN transistor Q1. The collector of transistor Q1 is connected to the power supply, and the emitter of transistor Q1 is connected to ground through a pair of serially-connected, forward-biased diodes D3, D4 and a resistor R3. The first side of the tank circuit is connected to the base of a second NPN transistor Q2 which is essentially connected in parallel with transistor Q1. Thus, the collector of transistor Q2 is connected to the power supply, and the emitter of transistor Q2 is connected to ground through a pair of forward-biased, serially-connected diodes D5, D6 and a resistor R4.
The first side of the tank circuit is also connected to the collector of a third NPN transistor Q3. The base of transistor Q3 is connected to the junction of diode D4 and resistor R3. The emitter of transistor Q3 is connected to ground through series-connected resistor R5 and constant current source IO1.
The collector of a fourth NPN transistor Q4 is connected to the second side of the tank circuit through series-connected NPN transistor Q5. The base of transistor Q4 is connected to the junction of diode D6 and resistor R4. The emitter of transistor Q4 is connected to ground through a series combination of resistor R6 and constant current supply IO2. The emitter of transistor Q4 is also connected to the emitter of transistor Q3 through a resistor R7.
Transistor Q5 constitutes a phase adder circuit. Applied to the base of the transistor Q5 is a fixed bias from a power source B1. Applied to the emitter of the transistor Q5 is a current signal Vs which is derived by phase-shifting the oscillation output of the oscillator circuit by 90.degree. and multiplying the phase-shifted signal by a modulating signal such as an audio signal.
The frequency modulator constructed as above, however, has the following problems.
(1) Since the transistor Q5 of the phase adder circuit for varying the phase of the oscillation of the oscillator circuit is connected in series with the transistor Q4 of the oscillator circuit, the power source voltage Vcc must be high to ensure proper operation of the transistors Q4, Q5.
(2) Since the signal Vs is given in the form of a current signal (one that is given by a current source), the voltage at the emitter of the transistor Q5 cannot be made low.